Don’t unnecessarily update files on file systems with lower mtime res…

…olutions

Fixes #74

beetsplug/alternatives.py

@@ -193,6 +193,12 @@ def item_change_actions(self, item, path, dest):
             actions.append(Action.MOVE)
 
         item_mtime_alt = os.path.getmtime(syspath(path))
+        if item_mtime_alt % 1 == 0:
+            # On Some filesystems (e.g. FAT32) the modification time resolution is
+            # a second. This means external files can look one second older than
+            # they actually are, which leads to unnecessary writes.
+            item_mtime_alt += 1
+
         if item_mtime_alt < os.path.getmtime(syspath(item.path)):
             actions.append(Action.WRITE)
         album = item.get_album()

test/cli_test.py

@@ -273,7 +273,7 @@ def test_update_older(self):
         mediafile = MediaFile(syspath(self.get_path(item)))
         self.assertEqual(mediafile.composer, "JSB")
 
-    def test_no_udpdate_newer(self):
+    def test_no_update(self):
         item = self.add_external_track("myexternal")
         item["composer"] = "JSB"
         item.store()
@@ -284,6 +284,26 @@ def test_no_udpdate_newer(self):
         mediafile = MediaFile(syspath(self.get_path(item)))
         self.assertNotEqual(mediafile.composer, "JSB")
 
+    def test_no_update_mtime_second_resolution(self):
+        """Assert that we don’t update the external file if the file system it is
+        on has a modification time resolution of only a second."""
+        item = self.add_external_track("myexternal")
+
+        mtime = 10000.5
+        os.utime(item.path, (os.path.getatime(item.path), mtime))
+        # Set modification time of external path to the integral part of the
+        # actual modification time. This makes the file appear older than it
+        # actually is.
+        os.utime(self.get_path(item), (os.path.getatime(item.path), int(mtime)))
+
+        item["composer"] = "JSB"
+        item.store()
+
+        self.runcli("alt", "update", "myexternal")
+        item.load()
+        mediafile = MediaFile(syspath(self.get_path(item)))
+        self.assertNotEqual(mediafile.composer, "JSB")
+
     def test_move_after_path_format_update(self):
         item = self.add_external_track("myexternal")
         old_path = self.get_path(item)

test/helper.py

@@ -320,8 +320,10 @@ def add_external_album(self, ext_name, **kwargs):
         album.load()
         return album
 
-    def get_path(self, item, path_key="alt.myexternal") -> Optional[bytes]:
-        return alternatives.External._get_path(item, path_key)
+    def get_path(self, item, path_key="alt.myexternal") -> bytes:
+        path = alternatives.External._get_path(item, path_key)
+        assert path is not None
+        return path
 
 
 class MockedWorker(alternatives.Worker):